1. Field of the Invention
This invention relates to disk drives. More particularly, this invention relates to a head stack assembly having a unitary molded plastic E-block and a hard disk drive including the head stack assembly.
2. Description of the Prior Art and Related Information
A huge market exists for disk drives, e.g., hard disk drives, for mass-market host computer systems such as servers, desktop computers, and laptop computers. To be competitive in this market, a hard disk drive must be relatively inexpensive, and must accordingly embody a design that is adapted for low-cost mass production. In addition, it must provide substantial capacity, rapid access to data, and reliable performance. Numerous manufacturers compete in this huge market and collectively conduct substantial research and development, at great annual cost, to design and develop innovative hard disk drives to meet increasingly demanding customer requirements.
Each of numerous contemporary mass-market hard disk drive models provides relatively large capacity, often in excess of 1 gigabyte per drive. Nevertheless, there exists substantial competitive pressure to develop mass-market hard disk drives that have even higher capacities and that provide rapid access. Another requirement to be competitive in this market is that the hard disk drive must conform to a selected standard exterior size and shape often referred to as a "form factor." Generally, capacity is desirably increased without increasing the form factor or the form factor is reduced without decreasing capacity.
Satisfying these competing constraints of low-cost, small size, high capacity, and rapid access requires innovation in each of numerous components and methods of assembly including methods of assembly of various components into certain subassemblies. Typically, the main subassemblies of a hard disk drive are a head disk assembly and a printed circuit board assembly. The printed circuit board assembly includes circuitry for processing signals and controlling operations of the drive.
The head disk assembly includes an enclosure including a base and a cover, at least one disk having at least one recording surface, a spindle motor for causing each disk to rotate, and an actuator arrangement. Actuator arrangements can be characterized as either linear actuators or rotary actuators; substantially all contemporary, cost-competitive small form factor drives employ a rotary actuator.
Typically, a rotary actuator arrangement includes a permanent magnet arrangement, a head stack assembly, and a pivot bearing cartridge for rotating the head stack assembly. The head stack assembly includes a coil frame including a coil, a body portion which surrounds the pivot bearing cartridge, a plurality of actuator arms ("arms") attached to the body portion, and a head gimbal assembly attached to each arm. The head gimbal assembly includes a load beam, a gimbal attached to the load beam, and a head supported by the gimbal. The head is positioned over a track on a recording surface of a disk to read or write data from or on the track.
Typically, the body portion and the arms of the head stack assembly are made out of metal and forms a unitary structure known as an "E-block." A plastic coil frame may be overmolded around the E-block. The E-block typically includes three arms but also may include other number of arms such as two, four, five and six arms. Such an E-block may be made by a combination of processes such as cast and machining processes which tend to be costly. In addition, such an E-block has a relatively high mass since the entire unitary structure is made out of metal. The relatively high mass results in a corresponding high moment of inertia about the pivot axis of the head stack assembly. Such a high moment of inertia results in relatively high access times for the disk drive for a given amount of power applied to the coil.
The high access times are also dependent on vibrations induced in the head stack assembly. These vibrations can include frequency components at or near resonant frequencies of the head stack assembly, resulting in relatively high amplitude vibrations. Such vibrations can degrade the performance of the disk drive, especially if the vibrations occur at frequency components substantially below a servo sampling rate of the disk drive. For example, FIGS. 10A-10B are graphs which show the effects of resonance for a Prior Art magnesium E-block in a disk drive having a servo sampling rate of 4.3 kilohertz (kHz). As shown, a dominating peak 700 occurs at about 3.6 kHz which may adversely affect the servo system of the disk drive since peak 700 may reduce the gain and phase margin of the servo system.
In efforts to reduce the mass of the E-block, some head stack assemblies employ a "hybrid" structure which includes a plastic overmolded structure encapsulating a plurality of stamped individual metal arms. The plastic overmolded structure includes a plastic overmolded body portion and a plastic overmolded coil frame. However, such hybrid structures are also costly to make since, inter alia, the arms are formed by a stamping process.
As mentioned above, vibrations can occur at frequency components at or near resonant frequencies of the head stack assembly. One such resonant frequency ("overmold interface resonant frequency") is defined by an interface between the plastic overmolded coil frame and the all metal E-block or between the plastic overmolded body portion and the stamped metal arms. In each instance, the plastic suitably includes a thermoplastic material such as PPS having glass filler material. When plastic is overmolded around an all metal E-block or stamped metal arms, the overmolded plastic clamps around cleated portions of the all metal E-block or stamped metal arms. While the plastic is adequately attached to the metal parts such as the all metal E-block or stamped metal arms, it is difficult to accurately control the location of the overmold interface resonant frequency such that it falls on or near the servo sampling rate. Such difficulty arises since the location of the overmold interface resonant frequency is determined by numerous variables such as surface finish of the metal parts, mold temperature and pressure, glass filler material content and quality, and moisture content of the thermoplastic material.